The present invention relates to improvements to optical fiber image conductors. At present, an image conductor is formed by grouping together a large number of very fine optical fibers whose diameters are between 10 and 12 microns, arranged in an orderly and coherent fashion at both ends of the cable thus formed. On one of the ends of this cable is projected the image to be transmitted, each fiber conducting a light point of the image which is thus transmitted and seen at the other end of the cable by means of an eye-piece.
This type of image conductor operates fairly correctly but the correct orderly arrangement of the fibers forms a considerable disadvantage and a great constraint. In fact, grouping the different fibers together at both ends leads to distortion of the received image.
Multimodal fibers are also known which are used at the present time for transmitting information by modulating the power of the optical wave.
The Applicant has surprisingly discovered that advantage may be taken of the modal architecture of this wave and that it is perfectly possible to use the properties of index gradient and high transmission power fibers.
With this process of modulation, the transmission band of such a fiber may reach several hundred MHz.km or even exceed a GHz.km. When the spectral width of the source is sufficiently small, the limiting phenomenon of the pass-band is the modal dispersion. Now, this modal dispersion does not occur in unimodal fibers whose pass-bands exceed a hundred or so GHz.km. The passband limitation is due to the dispersion of the optical guide and to the polarizing dispersion.
Thus, the Applicant has discovered that in an index gradient multimode fiber, the unimode propagation of one mode takes place without appreciable coupling with the other modes of the fiber.
He has thus discovered that each mode of a fiber is capable of conveying the same amount of information as a unimode fiber, providing that the mode coupling in the fiber is low and that the different modes are excited and recovered separately at the ends of the fiber.
In the experiment that the Applicant has carried out, the modal configuration chosen was a tubular mode for which the light energy remained confined in a region situated at a fixed distance from the axis of the fiber, the light transmitting rays being helical rays having the same axis as that of the fiber.
In this experiment, the source was formed by a nonpolarized He-Ne laser of wavelength 0.633 micron. The beam expanded by an afocal assembly passed through a modal filter formed from an opaque plate of 5 mm with perforations situated at its periphery, said plate being clamped between two glass plates so as to form optical cavities one out of two of which is at a pressure slightly less than the atmospheric pressure to create a phase shift equal to TL between the rays which have passed through the cavities.
The image of the filter is projected on to the input face of the fiber with the axis of the beam parallel to that of the fiber.
With a fiber 50 meters in length, the image at the output of the fiber is very little deformed with respect to the image projected at the input, which shows that over this distance, the mode coupling is very low.
This experiment shows that monomode injection and propagation are possible in a highly multimodal index gradient fiber.
It is on the basis of this characteristic of the index gradient fiber that the Applicant has perfected an image conductor which answered better the requirement of practice than previously known image conductors.